This invention relates to a tape cassette such as a digital video cassette which is used in a magnetic recording and reproducing apparatus, and more particularly to a tape cassette provided with an openable lid member in front of the cassette for protecting a tape medium.
In a conventional tape cassette of this type, a front lid having an L-shape in cross section is formed with a concave or a convex on its top plate, thereby to prevent a deflection of the front lid in a longitudinal direction caused by a shrinkage of a resin on occasion of injection molding (Post-examined Japanese Utility Model Publication 4-11271).
However, in case of the above described conventional tape cassette, even though the longitudinal deflection of the front lid can be prevented, side plates formed on opposite sides of the front lid and an upper lid constituting the lid member cannot be prevented from leaning due to the shrinkage of the resin.
Referring to FIGS. 4A and 4B, this leaning phenomenon will be explained.
FIG. 4A is a bottom view of the front lid in the conventional tape cassette, and FIG. 4B is an enlarged view of a joint portion between the side plates and the front plate of the front lid. As shown in FIG. 4A, the front lid 41 includes a front plate 42 covering the front face of the tape and a top plate 43 extending from an upper edge of the front plate 42 in an almost right angle. At opposite ends of the front plate 42 and the top plate 43 having a cross section of an L-shape, are integrally formed side plates 45 provided with sub shafts 44 which act as rotating axes of the front lid when it is opened or closed.
As shown in FIG. 4B in an enlarged scale, the front plate 42 and the side plate 45 are formed thicker in areas adjacent to the joint portion P1 therebetween than in other areas, because they constitute a corner zone, and thus, hardening after the injection molding proceeds more slowly at the joint portion than in surrounding areas. Accordingly, while the area adjacent to the joint portion P1 is hardened, the front plate 42 and the side plate 45 which have already been hardened are pulled in directions of arrows (a) and (b) by a shrinking force of the resin. For this reason, the side plate 45 inclines in a direction of an arrow (c) to come into a position as shown by a two-dot chain line, so that the front plate 42 and the side plate 45 may not intersect at a right angle.
In case where the side plate 45 inclines in this manner so that the front plate 42 and the side plate 45 may not intersect at a right angle, the front lid 41 cannot be smoothly opened or closed when it is fitted to the case body to be opened or closed, because the sub shaft 44 inclines or an inner face of the side plate abuts on an outer side face of the case body. Moreover, the shrinking force exerted as described above when the resin is set is likely to create sink marks around the joint portion P1 between the front plate 42 and the side plate 45, which will degrade strength as well as an outer appearance of the tape cassette.
Although not specifically illustrated, the above described problem may occur not only in the front lid, but also in the upper lid which is adapted to be opened or closed in association with the front lid.